Graphene Film: Strong or Fragile?

Some researchers claim graphene is the most important substance since the first synthetic plastic was invented. Graphene is a thin film of pure carbon that can be even one-atom thick. The carbon atoms inside are arranged in a flat and hexagonal structure which looks like honeycombs or chicken wires. Thickness of a single graphene flake is only 1 nm, less than 0.0000001 inches! Even a cling film is too thick to compare with it. Graphite is a common example built up by layers of graphenes, where the layers are loosely linked. Another exactly opposite allotrope is the carbon fiber, where the links between the graphene layers are irregular enough to prevent slippage and therefore increase the material intensity.

Graphene is considered extremely tough. It is said that a graphene sheet as thin as a cling film is able to support an elephant. It can be true, but unrealistic so far. A research, conducted in 2008 at Columbia University in the US, reported that its extraordinary strength could be ascribed to the robust carbon-carbon covalent bond and, more importantly, no microscopic defects because defects are inevitable in essentially all materials.

The intrinsic strength is the critical or maximum stress that a defect-free material withstands just before its atoms are pulled apart at the same time. As determined by the research team, the intrinsic strength of defect-free graphene is 42 N/m. If a cling film, typically ca. 100 µm thick, were to have the same strength as a perfect graphene layer, it would need as much a force as the weight of a large car, to pierce it with a pencil. In spite of this, defects like cracks or scratches are "weaker" microscopic structures that unbalance the inner stress of graphene. Number and sizes of defects unfortunately ruin its intrinsic strength although it is the stiffest known material - even stiffer than diamond.

In fact, the monolayer graphene becomes more fragile if it grows larger. Therefore, it is far from ideal for use as a macroscopic material. Numerous studies are trying to improve its production method for large-scale application. The aim is not only to produce larger flakes, but to cut them with extreme precision after it was first cut using an Atomic Force Microscope at an experimental level. To date, there are many techniques to manufacture larger and stronger graphene such as reduction of graphite oxide or silicon carbide, sodium ethoxide pyrolysis, nanotube slicing, solvent exfoliation and so forth. However, these methods vary in quality and quantity. A promising technique for making large pieces with best quality may be the silicon-based epitaxy technology announced in 2011.

Recently, graphene film is synthesized via a method called chemical vapor deposition (CVD). The carbon atoms are actually deposited on a substrate one after another to form the film. CVD enables a higher quality with some imperfections though. Nonetheless, this process is very expensive and thus impractical for commercialization. Surprisingly in September 2013, a start-up company called Graphene Frontiers launched a new manufacturing technique in a roll-to-roll process, which might support large-scale production of high-quality graphene. The company is trying to be a pioneer adapting CVD to a roll-to-roll process. Anyway, it is a good start despite some potential downstream obstacles to challenge Graphene Frontier.

In the foreseeable future, graphene will be hopefully used to fabricate strong products like helmets. The mission right now is to make this potent material inexpensive and bulky for market acceptance.